Spinet and upright pianos both fall into the category of vertical pianos, which have the inside strings and soundboard perpendicular to the ground, as opposed to the horizontal string and soundboard layout inside of a grand piano. Vertical pianos (which include spinets, consoles, studios, and uprights) are ideal for pianists who want to save space and money, though the quality of tone is not as bold or resonant as grands, because of their smaller size and their inability to aim the sound of the instrument towards other listeners in the room when the top is opened.
Vertical pianos in general came into existence because of a desire for a size more manageable than grands. Upright pianos (the largest of the verticals) were highly popular in the United States from the 1920s to the 1940s, but still a smaller version of the instrument was desired. The toy-sized spinet model (the smallest of the verticals) came into popularity during the Great Depression, but was mostly discontinued in the 1990s because of its inferior sound quality.
The height of the verticals is measured from the bottom of the back to the top of the back of the instrument. Spinets can range from a height of 36 inches to 40 inches, while uprights can range from 45 inches or higher.
The upright model (like the console and the studio) employs the vertical hammers on the inside of the instrument to directly strike the soundboard in the front when the keys on the keyboard are pressed. Spinets, on the other hand, have what is referred to as a "dropped action," which requires an additional lever system on the inside of the piano to hit the soundboard from behind instead of directly.
Upright pianos require regular maintenance, especially in regards to tuning. Should the instrument not be properly maintained, all might as well be lost on the quality of sound. Spinets can be especially difficult to tune because of the short string lengths and small soundboard area, which do not provide a particularly resonant tone. Also, the "dropped action" of the spinet results in a bit of a delay after playing the keys on the keyboard. In fact, many pianists recommend a console model instead, which is almost as small as the spinet (40 to 43 inches tall), but enforces a direct action.